This invention relates to a method of manufacturing a rotogravure printing medium and more particularly, to a method of applying a plastic printing medium to a printing roll or cylinder which is employed in rotogravure printing.
The present invention is related to commonly assigned U.S. patent application Ser. No. 514,595, filed Apr. 26, 1990, abandoned, and to commonly assigned, co-pending U.S. patent application Ser. No. 691,693 filed Apr. 24, 1991, abandoned, both of which are incorporated by reference hereinto. The subject matter of these applications later matured into U.S. Pat. No. 5,694,852 which is based on a continuation in part application of Ser. No. 08/525,880 filed Sep. 8, 1995, abandoned, which is a continuation in part of Ser. No. 991,499 filed Dec. 17, 1992, abandoned, which is a continuation of Ser. No. 691,693 noted above.
Rotogravure printing is a generally conventional method of printing on a sheet, web, or other substrate. The substrate may be a coated, uncoated, or metallized paper; glassine; plastic films and sheets made from vinyl, cellulose, acetate, polyester and polyethylene; plastic shrink films; paperboard; aluminum foil; fabrics; and similar materials. Rotogravure printing is capable of reproducing both subtle shades of color and black and white, and is particularly well suited for printing great numbers of copies precisely and rapidly. Typical end products for the printed substrates include labels, cartons, paper and plastic cups, trading stamps, wrapping paper, and sheet vinyl flooring.
Rotogravure printing is the only commercial printing process which can control both ink thickness and the area of ink coverage. This is achieved by etching or engraving recessed microscopic wells, frequently referred to as xe2x80x9ccells,xe2x80x9d of varying depth and area in a printing medium or image carrier surface. In controlling the size and depth of the cells, the amount of ink available for placement on the substrate is controlled to generate an image composed of an arrangement of large and small dots. Other types of printing, such as flexographic printing, are generally similar to rotogravure printing, but are specifically different, e.g., as to thickness of the printing medium and the character and formation of ink-transferring surfaces.
In typical rotogravure printing, the printing medium or image carrier is a copper film electro-deposited from a chemical bath on a specially prepared steel cylinder. Prior to the engraving of the recessed wells, the copper is mechanically ground and polished. After engraving, the cylinder requires the addition of plated, hard chromium for durability and wear resistance. During the printing process the cylinder is rotated in a bath of ink. Excess ink is wiped away by a doctor blade and the ink remaining in the engraved cells is then transferred to a substrate as discrete dots, while the substrate passes between the engraved, inked cylinder and a soft pressure roller. Rotogravure printing using non-copper printing media is similarly effected.
The recommended modem process to prepare a copper image carrier requires the use of electrolytic deposition from an acid/copper bath. A steel cylinder of the required diameter is partly immersed in a chemical copper solution and rotated at a regulated speed. An electrical current running through the cylinder and the solution gradually deposits a coating of copper on the rotating cylinder until the approximate required thickness is achieved. The copper plated cylinder is washed and then polished to final dimensions with a smooth, mirror-like surface finish.
The copper coating is then engraved, either chemically or electronically. In the chemical engraving process, cells are formed by acid etching of the copper coating. The cells are formed by a screen which prevents the acid from reaching selected portions of the copper surface. The resulting acid-etched wells are round in shape and slightly smaller at the bottom than at the top.
The process of forming the copper coating for the printing cylinder and of chemically engraving the copper coating may result in the formation of waste products which are environmentally hazardous, requiring costly disposal. Further, the prior art techniques are costly and time-consuming.
An object of the present invention is a method of manufacturing a rotogravure printing medium which is inexpensive and expedient to produce and which avoids other shortcomings attending the use of copper (or other metallic) printing media.
In accordance with an aspect of the present invention, there is provided a method of manufacturing a printing medium for application to a printing apparatus (e.g., a rotogravure printing drum or cylinder). The terms xe2x80x9cprintingxe2x80x9d and xe2x80x9crotogravure printingxe2x80x9d, as used herein, include any apparatus, device or method which involves the transfer of an inked image. The printing medium comprises a plastic composition which may be applied to a flat plate or a cylinder to form a plastic-coated printing plate, roll or cylinder, the plastic coating being etched, engraved or otherwise selectively removed to form the printing cells.
The plastic composition is any flowable, self-levelling, curable material capable of being deposited on a flat surface or on a printing cylinder or roll according to the method hereof to form a continuous coating, following curing of which, the composition may be etched, engraved or otherwise selectively removed to produce a printing surface. Preferred plastic compositions are those self-levelling, flowable materials set forth in the commonly assigned ""595 and ""693 applications.
The plastic composition may be applied to the printing substrate by various means well known in the art. The method of the present invention is particularly applicable to the application of the plastic composition in a flowable form to a printing roll or cylinder which is employed in a rotogravure printing process. The printing roll or cylinder may be made of a metal, such as aluminum, or steel, and may, contrary to the prior art, also be made of a non-metal, such as a plastic.
Prior to the application of the plastic composition to the printing roll or cylinder, the printing roll or cylinder may be pretreated by means of a plasma or corona pretreatment to clean and/or alter the surface (i.e., lower the surface tension) of the cylinder or roll for improved film or coating wetting and bonding strength.
When a corona pretreatment of the surface of the printing roll or cylinder is employed, the surface thereof may be treated with an accurately-directed electrical bombardment of the surface to clean and/or alter the surface of the printing roll or cylinder.
When an aluminum printing cylinder is employed, the surface may be pretreated so as to provide an anodized surface. When a steel cylinder is employed, the cylinder may be treated with an oxide such as black oxide.
Methods of applying the plastic composition include spraying the composition onto the surface of the printing substrate such as the printing roll or cylinder. Such spraying may be accomplished through the use of a nozzle through techniques known in the art. Other methods which may be employed include dip coating, spin coating, and ring coating. The coating, upon application by any method to the surface of the printing substrate intended for use in rotogravure printing, preferably has a thickness of from about 0.003xe2x80x3 to about 0.015xe2x80x3. Where the printing substrate is to be used for other types of printing, such as flexographic printing, thickness up to about 0.040xe2x80x3 or more.
The preferred method of applying a selected plastic composition to the printing roll or cylinder is that described in detail below. Any of the compositions disclosed in the above-noted ""693 application, as well as other curable, flowable, self-levelling plastic compositions may be applied to the printing roll or cylinder. Included are compositions in which printing images are xe2x80x9cdevelopedxe2x80x9d following selective exposure to light or other radiation.
The selected plastic composition is applied to the printing cylinder by a delivery facility, such as a piston-cylinder, a metering pump or a precision gear pump from a defined site, such as a small orifice. If the composition comprises several materials, these may be mixed by static tube, mechanical or impingement techniques at or near the orifice. Preferably, the orifice is elliptical and is formed by angularly truncating a right circular cylindrical tube having a small circular cross-section bore to form a tip and a heel on the tube. The diameter of the bore, and the minor axis of the elliptical orifice, when viewed normally to the plane thereof, is about 0.010xe2x80x3 to about 0.055xe2x80x3, and is preferably about 0.030xe2x80x3. The major axis of the elliptical orifice, when viewed normally to the plane thereof, is about 4 to 8 times larger than the minor axis, that is, about 0.040xe2x80x3 to about 0.440xe2x80x3, and is preferably about 0.120xe2x80x3 to about 0.240xe2x80x3.
The plastic composition is applied through the orifice as the orifice and surface are relatively moved. Where the surface is on a cylinder, is preferably rotated as the tube is linearly moved or scanned across the rotating surface thereof The plane of the elliptical orifice is tangentially proximate to the printing cylinder along the minor axis thereof (i.e., about midway between the tip and the heel), with the major axis extending along the direction of printing cylinder rotation. The plane of the orifice is preferably slightly upwardly tipped in the direction of movement of the tube. The plastic composition, when applied to the printing roll or cylinder, has a viscosity of from about 800 cP to about 5,000 cP, the viscosity preferably being from about 1,000 cP to about 2,000 cP. The plastic composition is applied at a pressure of from about 8 psi to about 60 psi, preferably at about 30 psi. The printing cylinder may be of a standard size, for example, it may have a diameter of about 361 mm, and is rotated at speeds of about 30 to about 90 rpm, with about 45 rpm being preferred. The tube and its orifice are moved along the rotating cylinder""s surface at a rate of from about 0.008xe2x80x3 per revolution to about 0.048xe2x80x3 per revolution, with about 0.0192xe2x80x3 per revolution being preferred.
If desired, multiple orifices may be used to deposit the plastic composition in several streams.
The orifice area, the viscosity of the plastic composition, the pressure at which the plastic composition is applied, the cylinder rotational speed, and the rate of movement of the tube and orifice across the cylinder surface are adjusted such that when the plastic composition is applied to the printing roll or cylinder, the thickness of the plastic composition deposited upon the cylinder is from about 0.003xe2x80x3 to about 0.015xe2x80x3, preferably from about 0.0032xe2x80x3 to about 0.0035xe2x80x3, and most preferably at about 0.0035xe2x80x3. The plastic composition preferably is applied to the printing roll or cylinder at room temperature (about 23xc2x0 C.), while the printing roll or cylinder, prior to application of the plastic composition, may be preheated to a temperature of from about 23xc2x0 C. to about 40xc2x0 C., preferably to about 30xc2x0 C. It is preferred that the plastic composition be deposited to a desired thickness in a single pass of the tube and orifice across the surface of the rotating printing roll or cylinder.
In applying the plastic composition to the printing roll or cylinder as described above, there is formed a helical strip or bead of the plastic composition, the strip or bead having a circular and/or lobate cross-section as it is deposited on the printing roll or cylinder. Deposited portions of the helical strip or bead began to self-level, and adjacent, deposited portions of the strip or bead merge after being deposited to become a continuous coating of substantially uniform thickness and having the preferred average thickness on the printing roll or cylinder. Where the surface is planar, the adjacent portions of the strip or beed may be formed by indexing the tube or by the use of multiple orifices.
Following curing of the continuous coating, it is capable of being engraved, etched or otherwise selectively removed (as by photographic-like development or laser scribing) to provide a printing surface. Methods of curing include, but are not limited to, ultraviolet irradiation (which may be followed by heating), heating, and gelation at room temperature. The method employed to cure the composition depends upon the particular plastic composition applied to the printing substrate.
After the plastic coating is applied to the substrate and cured, it is engraved or etched so as to provide a printing medium or image carrier. The engraving may be accomplished by any of various engraving or etching methods known in the art; however, a preferred method of engraving is electronic engraving. Electronic engraving may, in one embodiment, be carried out using a diamond stylus which has an included angle of from about 110xc2x0 to about 130xc2x0. The narrower the included angle, the deeper the stylus cuts into the plastic coating. As the stylus cuts into the coating, it forms a plurality of wells in the coating. Each well has an angled wall, and is smaller at the bottom than at the top.
The reliability of electronic engraving can be enhanced by employing an air knife device to aid in the removal of chips away from the support, or foot, of the diamond stylus. The air knife dispenses a precise, focused, and continuous or pulsed air stream. The air stream moves in a direction opposite that of the movement of the cylinder. The air stream directs chips away from the support, or foot, of the diamond stylus, the cutting diamond, and the burr cutter in a direction toward a vacuum device, whereby the chips may be removed from the printing surface by the vacuum device located in the cutting head.
Prior to engraving, the plastic coating may be contacted (preferably by spraying) with a finely divided fluropolymer as a dry film lubricant for the plastic coating. The dry film lubricant provides for lubrication of the support, or foot, of the diamond stylus as the stylus traverses the plastic coating during the engraving. Such lubrication provides for improved penetration of the surface of the plastic coating by the diamond stylus and provides for increased life of the diamond stylus. A preferred finely divided fluoropolymer powder is a micronized tetrafluoroethylene powder. An example of such a micronized tetrafluoroethylene powder is sold by DuPont, Wilmington, Del., as Vydax.
Once the printing medium or image carrier is formed on the substrate, it is ready for the application of printing ink. Examples of gravure type inks which may be applied to the printing medium include aliphatic hydrocarbon inks such as A-Type inks and B-Type inks; nitrocellouse inks (C-Type); polyamide inks (D-Type); alcohol-based inks (E-Type); polystyrene-based inks (M-Type); chlorinated rubber-based inks (T-Type); vinyl chloride or vinyl acetate copolymer-based inks (V-Type); inks employing water as a solvent base (W-Type); X-Type inks including heat transfer and sublimation inks; and foam inks. Preferred inks are those of the A, B, C, D and T Types. The type of ink employed depends upon the type of surface that is to be printed. Upon application of the ink, any excess ink is removed by a doctor blade. It has been found that a doctor blade formed from a polymer such as polyester, nylon, polyethylene, polypropylene, or polyacetal, and having a tapered edge which contacts the printing medium, conditions the image-bearing surface without substantial wear and is a great improvement over metal doctor blades employed in the art with copper-etched surfaces. Most preferably, a polyester doctor blade is employed for removing the excess ink. Examples of polyester doctor blades having a tapered edge which may be employed in accordance with the present invention are those of Esterlam""s E350/E500 range of laminated polyester doctor blades, sold by Esterlam International Limited, of Devon, England.